1. Field of the Invention
The present invention relates to an infrared photodetector with improved or enhanced quantum efficiency.
2. The Prior Art
One of the primary objectives of the future projects for space-based observatories, such as the Space Infrared Telescope Facility (SIRTF) and the Large Deployable Reflector (LDR), is to make background-limited astronomical observations. In order to fully realize this objective, the requirements on detector characteristics, in terms of responsivity and noise equivalent power (NEP), have necessarily become more stringent. In addition, detectors will have to withstand the ionizing radiation environment of the high earth orbit. Several approaches have been adopted in the past to improve the fundamental characteristics of an infrared photoconductor. The quantum efficiency, which is perhaps the single most important parameter or defining characteristic, can be enhanced by increasing the absorption depth of the detector. An increased absorption depth can be achieved by geometrical means, by increasing the dopant concentration or by both. These approaches can have serious drawbacks, especially for low-background astronomy.
Increasing the dopant concentration will improve the absorption coefficient but, at the same time, will increase the leakage current. High leakage current and hopping conduction degrade the noise performance of the detector. In an impurity-band-conduction (IBC) detector, the attempt is made to overcome this problem by growing a high purity epitaxial layer on the top of the active layer. An IBC detector can, therefore, theoretically take advantage of a very high dopant concentration with improved NEP because of this blocking epitaxy. Although the IBC concept has been successfully demonstrated for mid-infrared silicon detectors, the technology for far infrared detectors is considerably less than optimum.
Given an acceptable dopant concentration and aside from increasing the physical length of the detector element, several geometrical schemes can be used to increase the optical length of a detector. One of the most common methods involves the utilization of an integrating cavity behind the detector to refocus the light that is not absorbed back onto the detector so as to increase the chances of the light being absorbed. In another approach, the exit end of the detector is beveled at the proper angle to induce total internal reflection. Both of these methods have been successful in increasing the quantum efficiency by up to a factor of 3. Although these approaches are easily implemented on a single discrete detector, such approaches pose a formidable engineering task where an integrated detector array is to be produced.